In U.S. Pat. Nos. Re. 31,968 and 4,523,051 (incorporated herein by reference), Michelsen and Chen disclose methods of making graded I-III-VI.sub.2 semiconductors having internal p-n type regions formed by controlling the evaporation of the elements in the ternary compound so as to form a graded resistivity caused by a graded composition in the semiconductor. For example, a graded CuInSe.sub.2 thin film semiconductor could be formed with two distinct regions: a copper-rich p-type region near a base contact, and a copper-deficient n-type region near the photoactive junction of the thin film. The cells that Mickelsen and Chen produced have photovoltaic conversion efficiencies approaching 10%. Surprisingly efficient cells can be made by optimizing the production process.